PTC or autotherming behavior is a characteristic of the resistivity of a composition at certain temperatures. A composition that exhibits PTC behavior has an R.sub.14 value of at least 2.5, where R.sub.14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, an R.sub.100 value of at least 10, where R.sub.100 is the ratio of the resistivities at the end and the beginning of a 100.degree. range, and the composition also preferably has an R.sub.30 value of at least 6, where R.sub.30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. A plot of the log of the resistance of an element composed of a PTC composition versus temperature will often show a sharp change in slope over a part of the temperature range in which the composition has an R.sub.100 value of at least 10.
Conductive polymer compositions exhibiting PTC behavior are especially useful when incorporated into electrical devices for applications that require self-controlling temperature. For example, the electrical devices may be heaters, circuit protection devices, or sensors and the applications may include utility equipment, telephone equipment, wires, cables, or computers.
The PTC composition art includes many blends of materials, but mostly all include a particulate conductive filler, such as carbon black, dispersed in a crystalline or semi-crystalline resin or matrix. The resin material is usually a polymer comprised of polyolefins, polyamides, polyimides, polycarbonates, fluoropolymers, or carbohydrates. The particulate conductive materials may consist of organics, inorganics, or organometallics.
In U.S. Pat. No. 5,174,924 to Yamada et al., a conductive polymer composition exhibiting PTC behavior is disclosed. This composition contains a crystalline polymer and a carbon black having an average particle diameter of at least about 60 millimicrons and a Dibutyl Phthalate (DBP) absorption of at least about 80 cc/100 g. Use of the particular carbon black in this composition contributes to the composition's high breakdown voltage.
U.S. Pat. No. 5,171,774 to Ueno et al. discloses a method for producing a PTC composition wherein the carbon black component is etched at a high temperature to increase its specific surface area. The resulting porous carbon black is then blended with a crystalline polymer to form a conductive polymer composition.
U.S. Pat. No. 4,818,439 and its related U.S. Pat. No. 5,143,649, both granted to Blackledge et al., reveal an electrically conductive polymer composition with PTC behavior comprising a finely divided electrically conductive carbon black dispersed within a low molecular weight polymer material and a method of manufacturing this composition. Inclusion of the low molecular weight polymer allows for the virtual elimination of the annealing step in the composition formation process.
In U.S. Pat. No. 5,106,540 to Barma et al., a series of conductive polymer compositions is disclosed. The filler in these compositions is itself a composite. The compositions may exhibit PTC, ZTC (zero temperature coefficient), or NTC (negative temperature coefficient) behavior, depending on the conductive filler which is used. Various components and various methods of making the compositions are disclosed.
It is an object of the present invention to teach a conductive polymer composition that is simple to make and form into useful devices and which exhibits a high degree of autotherming behavior.